Autonomic nervous system

The autonomic nervous system (ANS) is also called the vegetative nervous system.

It controls the involuntary functions and influences the activity of internal organs. The autonomic nervous system is regulated by the hypothalamus and is required for cardiac function, respiration, and other reflexes, including vomiting, coughing, and sneezing.

The autonomic nervous system can be divided into sympathetic and parasympathetic nervous systems.

Simply, the sympathetic and parasympathetic nervous systems have opposite actions.

Sympathetic fibres, located in spinal nerves are responsible for the "fight or flight" response, which is an acute response that takes place in case of an imminent harmful event or intense mental distress. To activate this response, the sympathetic fibres use the neurotransmitter noradrenaline to activate the blood flow in skeletal muscles and lungs, dilating lungs and blood vessels and raise the heart rate.

In other words, they initiate the physiological events that prepare the body for self-defence through a fight or an escape and therefore the type of synapses are of the excitatory type.

On the contrary, parasympathetic fibres regulate resting responses such as heart rate, salivation, lacrimation (secreting tears), digestion, with the only exception being sexual arousal. Parasympathetic motor fibres are found in four of the 12 pairs of cranial nerves.

So, synapses established by the parasympathetic fibres are typically inhibitory, with acetylcholine as main neurotransmitter.

Although most of the autonomic nervous system responses are involuntary, they can integrate with the somatic nervous system, which is responsible for the voluntary movements. For example, in the case of defecation, there is an interplay between voluntary and involuntary movements.


Author: Alessandra Donato from the Hilliard Lab

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